Pressure gradient transducer

ABSTRACT

A simple tubular housing structure has a vibratile diaphragm sealing each open end. Each of these diaphragms is mechanically connected to a separate vibratile piston. These pistons are rigidly bonded to the opposite sides of a piezoelectric transducer element assembly. These assemblies generate at least two independent voltages when sound pressure impinges upon the diaphragms. The voltages generated by each of these two piezoelectric plates are combined to effectively neutralize each other when sound pressures of the same amplitude and phase are simultaneously applied to each of the two pistons.

Unite States assa [ Feb.6,1973

[54] PRESSURE GRADIENT TRANSDUCER [75] Inventor: Frank Massa, Cohasset,Mass.

[73] Assignee: Massa Division, Dynamics Corporation of America, Hingham,Mass.

22 Filed: Feb. 19, 1970 21 Appl.No.:12,565

52 n.s.c|. .340/10,310/8.2,3l0/8.3,

W V 3l0/8.7 51 Int.Cl ..H04r17/00,H0lv 7/00 58 FieldotSearch..340/l0,8.7

[56] References Cited UNITED STATES PATENTS 3,187,300 6/1965 Brate..340/10 Primary ExaminerBenjamin A. Borchelt Assistant Examiner-H. A.Birmiel Attorney-Louis Bernat [57] ABSTRACT 11 Claims, 6 Drawing FiguresPRESSURE GRADIENT TRANSDUCER This invention relates to an improvedpressure gradient transducer, and more particularly to a transducerwhich may be used as a microphone either in the air or under water.

Reference is made to my earlier U. S. Pat. Nos. 3,354,426 and 3,363,228.Each of these patents shows a pressure gradient transducer of thedescribed type. However, they are much more complicated.

In each of these patents, there is shown a housing having a pair ofaxially aligned diaphragms sealing opposite ends thereof. A connectingrod extends between opposing sides of these two diaphragms. Apiezoelectric assembly includes a bilaminar flexural element having oneedge attached to the inside of the housing and another edge attached tothe connecting rod.

When the two diaphragms are exposed to a pressure field originating froma sound source lying in a plane perpendicular to the longitudinal axisof the connecting rod, the pressure on each diaphragm is of the same intensity and phase. Therefore, no differential force results and theoutput of the transducer is zero. For sounds arriving along an axisremoved from the perpendicular plane, a phase shift exists between thesound pressure arriving at each diaphragm. As a result, there is a netdifferential force which will displace the connecting rod. This rodmovement flexes the bilaminar assembly and causes a resulting signal tobe generated responsive to excursion of the piezoelectric element.

In the present invention, a pair of axially aligned vibratile pistonsare located at opposite ends of a hollow cylindrical housing structure.A piezoelectric element is bonded between the end faces of the opposingpiston structures. This element is capable of converting oscillatorycompressional forces to electrical signals. The configuration of thepiezoelectric transducer element is such that equal voltages aregenerated in two sections of the piezoelectric element assembly whenequal sound pressures of the same phase are imposed on each of theexposed piston surfaces.

An advantage of this arrangement is that the transducers of thisinvention are relatively simple, both in construction and operation. Theinventive transducers are manufactured easily and inexpensively.Nevertheless, they are rugged and reliable in operation, and theyprovide excellent response characteristics. They have relatively greatsensitivity, particularly within the audio frequency range.

According to an important feature of the invention, mechanicalconnection means are provided between two opposing vibratile diaphragms.These diaphragms sealingly close the opposite ends of a hollow tubularhousing. A compressionally sensitive piezoelectric structure inside thehousing generates at least two independently equal voltages when equalpressures, of the same phase, simultaneously impinge upon the exposedvibratile diaphragm surfaces.

An object of this invention is to provide means for combining theseindependently generated voltages to neutralize one to another anddeliver a zero voltage when equal pressures, of the same phase, impingeupon the exposed faces of the vibratile piston or diaphragm surfaces.

Another object is to provide new and improved transducers, of thedescribed type, which are simpler and lower in cost than similar priorart devices without sacrifice in performance.

These and other objects, features and advantages will become more fullyapparent from the following detailed description when taken inconjunction with the accompanying drawings in which:

FIG. 1 is an end view of a pressure gradient transducer incorporatingthe invention;

FIG. 2 is a cross-sectional view of the transducer, taken along the line22 of FIG. 1;

FIG. 3 is a cross-sectional view of another embodiment, showing atubular piezoelectric element that may be substituted for the tubularpiezoelectric element illustrated in FIG. 2;

FIG. 4 is a cross-sectional view of a central portion of atransducerassembly which may be substituted for the corresponding structure shownin FIG. 2;

FIG. 5 is a cross-sectional view of a modified portion of the structureshown in FIG. 4; and

FIG. 6 is a cross-sectional view of still another modification of theinner portion of the transducer assembly.

The major portions of the inventive pressure gradient transducer are ahousing 15, a pair of oppositely disposed transducer assemblies l6, 17,a driven piezoelectric element 18, and a waterproof cable 19.

The housing 15 includes tubular structure 20 which encloses a transducerassembly comprising a pair of tapered pistons 21 and 22. These pistonsare rigidly bonded to opposite ends of a piezoelectric cylinder 24. Thebonding may be made by an epoxy agent, or by any other suitable means.

A pair of small, center bosses 25 and 26 are located on the largediameter faces of the two tapered piston structures 21, 22,respectively. Thin walled caps 27, 28 enclose the two ends of thehousing. Each cap has a center hole in its face, which clears the bosses25, 26 to enable concentric alignment between the piston face and theperiphery of the cap. Each cap is bonded to the face of an individuallyassociated piston and sealed at its outer periphery to the housing 20.During operation of the transducer, the flat surfaces of the caps 27 and28 act as vibratile diaphragms which respond to the action of soundwaves striking their exposed outer surfaces.

The transducer and piston element assembly 18, inside the housing 15,comprises a number of electrode surfaces 30,31, 32 formed on thepiezoelectric ceramic cylinder 24. These electrode surfaces may be apair of fired silver layers applied to the inside and outside walls ofthe ceramic tube 24. A small uncoated margin is provided on the ceramiccylinder, at each end of the inside electrode 32. The two electrodes 30and 31 are symmetrical spaced around the outer periphery of the ceramiccylinder 24. When the piezoelectric ceramic tube 24 is polarized, onepolarity of the polarizing voltage is connected to the inside electrode32 and the two outside electrodes 30 and 31 are connected together andto the other polarity. The drawing has been marked, by way of example,to show that the electrode 32 is positive and the electrodes 30, 31 arenegative during polarization.

For wiring the piezoelectric ceramic element to the cable 19, anelectrical conductor 33 is soldered between the electrode 30 and thecenter conductor of the coaxial cable 19. Another electrical conductor34 is soldered between the electrode 31 and the shield of the coaxialcable 19. After the electrical connections are made the end of thecoaxial cable 19 is anchored by tape, or other means, (not shown) to theperiphery of one of the pistons, such as piston 22.

To complete the assembly, the transducer elements are placed inside thehousing 15. The coaxial cable 19 is brought out through a suitableopening in the wall of the housing tube 20. The end caps, or vibratilediaphragms 27 and 28, are then attached to the piston surfaces andsealed to the open ends of the housing tube 20.

During operation of the transducer, equal voltages are generated in eachhalf of the piezoelectric cylinder 24 when sound arrives along an axisperpendicular to the "axis of the cylinder 20. The voltage appearingacross the coaxial cable conductors is the'difference' between these twoequal voltages generated in the two separate electrode sections 30, 31of the ceramic cylinder 24. Thus, the transducer has a zero sensitivityto sounds arriving in a plane which is perpendicular to the longitudinalaxis of the housing. The transducer response is maximum when soundsarrive along the normal axis of the transducer assembly. In thisdirection there is a maximum phase shift of the pressure gradientappearing at each of the two diaphragms 27 and 28. Therefore, thedirectional characteristic of the complete transducer assembly, shown inFIG. 2, is a true cosine pattern, as illustrated in FIG. 9 of U. S. Pat.No. 3,363,228.

The transducer has a maximum sensitivity when the length of the housing20, along the longitudinal axis of maximum response, is equal toone-half the wavelength of sound at a preferred frequency in thepertinent medium. Below this preferred frequency, the sensitivity fallsoff, gradually at first and then at the rate of 6 dB per octave. Thischaracteristic in the response of the pressure gradient transducer isdescribed in connection with FIG. 8 ofU. S. Pat. No. 3,363,228.

FIG. 3 illustrates another embodiment of the electrode surfaces on thewalls of the piezoelectric ceramic cylinder. Here, there are twoseparate electrodes 40 and 41 on the inner surface of the piezoelectricceramic cylinder 42. Four separate annular electrodes 43, 44, 45, and 46are distributed over the outer surface of the ceramic cylinder 42.During polarization of the ceramic element 42, one polarity is connectedsimultaneously to the electrodes 40 and 41. The other polarity issimultaneously connected to all of the external electrodes 43, 44, 45,and 46.

If the ceramic tube 42 (FIG. 3) is substituted for the tube 24 (FIG. 2),the leads 33 and 34 are connected to the electrodes 43 and 46. A thirdelectrical conductor 47 connects the two electrodes 44 and 45 together.The electrode configuration illustrated in FIG. 3 approximately doublesthe sensitivity of the structure of FIG. 2.

Another embodiment replaces the piezoelectric structure 18 with thestructure which is illustrated in FIG. 4. Here, a modified pistonstructure 50, 51 is substituted for the piston structure 21, 22. Twoidentical piezoelectric discs 52, 53 are bonded to each other with aconducting cement, such as epoxy with silver granules. The oppositesides of the disc are bonded to the opposing ends of the two pistonstructures 50, 51. Electrically conductive foils 54, 55 and 56 areplaced between the cemented faces to provide for making externalconnections to the electrode surfaces of the piezoelectric discs.

These electrode surfaces are formed on the flat surfaces of the discs 52and 53 prior to the polarization thereof. After polarization, the discsare assembled with the polarity of one disc, 53, in contact with thepolarity of the other disc 52. These two surfaces of opposite polarityare commonly connected by foil electrode 56. The two outer electrodesurfaces of the discs 52, 53 are connected together by foil electrodes54 and 55. The conductor 33 is connected to the foil electrode 56, andconductor 34 is connected to foil electrodes 54 and 55, to complete anassembly which is fully substitutable for the assembly of FIG. 2.

FIG. 5 illustrates yet another embodiment for mountand 61 are bondedtogether with electrically conducting cement. The negative potentialsurfaces of these two discs are joined together. The outside or positiveelectrode surfaces of discs 60 and 61 are mechanically attached, one toeach of the two opposing surfaces of pistons 62a, 63a. Foil electrodes64 and 65 make electrical contact with the outer electrode surfaces ofthe discs 60, 61. For the arrangement of FIG. 5, either the pistons 62aand 63a must be electrically non-conductive or they must be insulatedfrom the electrically conducting foils 64 and 65. This insulation may beaccomplished by covering the ends of the pistons 62a, 63a, with asuitable material, such as a thin Bakelite sheet, before attachment todiscs 60, 61.

FIG. 6 shows still another embodiment of a transducer assembly that maybe used in place of the structure 18. In this arrangement, the ceramicdiscs 70 and 71 are separated from each other. An electricallyconductive, cylindrical rod 72 is bonded, with electrically conductingcement, between the positive potential face of the piezoelectric disc 71and the negative potential face of the piezoelectric disc 70. Foilelectrodes 73 and 74 provide electrical contact between the electrodefaces of the piezoelectric discs 70, 71 and the ends of the opposingpistons 75 and 76.

In this FIG. 6 embodiment, it is possible to use electrically conductivepistons. When these pistons are at tached to the piezoelectric discs 70,71 with an electrically conductive cement, the pistons assume the commonpotential of the electrode surfaces of the foil electrodes 73 and 74. Anelectrical conductor 77 makes an electrical connection between the foilelectrodes 73 and 74. Electrical conductor 34 connects the conductor 77to the shield of coaxial cable 19. Electrical conductor 33 establishesconnection between the common potential rod 72 and the center lead incoaxial cable 19; hence, the pistons 75 and 76 have the commonelectrical ground potential appearing on the shield of the coaxial cable19. Preferably, the cable 19 is fastened to the rod 72 by means of awrapping, such as tape 78 in order to provide strain relief and preventa tension on the connecting leads 33 and 34. If desired, it is possibleto insulate the pistons 75 and 76 from this common potential. Either thepistons 75, 76 are made from nonconductive material; or, an insulatingfilm is interposed at the point of contact between the pistons and theelectrode surfaces of the piezoelectric discs.

A further modification of the construction of FIG. 6 reverses thepolarity position of the piezoelectric disc 71. The negative electrodeson the two discs 70, 71 are then commonly connected by the rod 72. Ifthis polarity orientation is used, the pistons 75 and 76 must ing thepiezoelectric discs. The piezoelectric discs, W

remain insulated from the electrode surfaces of the piezoelectric discs,to which they are attached. Also, the electrical conductor 77 is notnecessary and may be eliminated. The electrical conductor 33 isconnected between the center conductor of the coaxial cable 19 and thefoil electrode terminal 73. Electrical conductor 34 is connected betweenthe shield of the coaxial cable 19 and the foil electrode terminal 74.This modified arrangement of FIG. 6 is similar to the arrangementillustrated in FIG. 5.

While several embodiments of the invention have been shown anddescribed, it should be understood that various modifications andalternative constructions may be made without departing from the truespirit and scope of the invention. Therefore, the appended claims areintended to cover all equivalent constructions which fall within theirtrue spirit and scope.

I claim:

ll. A pressure gradient electroacoustic transducer comprising atransducer element assembly having a pair of parallel surfaces, separatevibratile diaphragm means coupled to each of said parallel surfaces,means responsive to an application of substantially equal alternatingmechanical forces upon said diaphragms and said parallel surfaces forcausing said transducer element to generate two separate andsubstantially equal alternating voltages, and means for connecting saidtransducer elements for applying said separate and substantially equalalternating voltages in subtractive relationship one to the other,whereby an application of substantially equal mechanical forces uponsaid diaphragms produces a zero output.

2. The invention of claim 1 wherein said electroacoustic transducerincludes a tubular housing with an opening at each end of said tubularhousing, said transducer element assembly being positioned centrallywithin said tubular housing, and said separate vibratile diaphragm meansbeing attached to seal the open ends of said tubular housing.

3. An electroacoustic transducer comprising a tubular housing with anopening at each end of said tubular housing, a transducer elementassembly having a pair of parallel surfaces, said transducer elementassembly being positioned centrally within said tubular housing,separate vibratile diaphragm means coupled to each of said parallelsurfaces, said separate vibratile diaphragm means being attached to sealthe open ends of said tubular housing, said transducer element assemblyincludes a tubular piezoelectric element having electrode surfaces onboth the inside and outside peripheral walls of said tubularpiezoelectric element, means responsive to an application ofsubstantially equal alternating mechanical forces upon said diaphragmsand said parallel surfaces for causing said transducer element togenerate two separate and substantially equal alternating voltages,means for applying said separate and substantially equal alternatingvoltages in subtractive relationship one to the other, whereby anapplication of substantially equal mechanical forces upon saiddiaphragms produces a zero output.

4. The invention of claim 3 wherein at least one of said electrodesurfaces on one of said peripheral walls of said tubular element isdivided into two substantially equal and electrically se arated areas.

5. The invention of 0 arm 4 further characterized in that said voltageapplying means is arranged to apply the voltages generated in each ofsaid electrically separated areas in a subtractive relationship of onewith respect to the other.

6. The invention of claim 3 further characterized in that the electrodesurface on one peripheral wall of said tubular piezoelectric element isdivided into two electrically separated areas of substantially equalsize and still further characterized in that the electrode surface onthe opposite peripheral wall is divided into four separated areas ofsubstantially equal size.

7. The invention of claim 6 further characterized in that the wall ofsaid tubular piezoelectric element is polarized in the same directionthroughout the entire length of the tube.

8. The invention of claim 7 further characterized in that four separatevoltages are generated between the two separated electrodes on oneperipheral wall and the four electrodes on the opposite peripheral wallof the piezoelectric tube, and means for combining the four voltages sothat each is subtractive one from the other.

9. The invention of claim 1 further characterized in that saidtransducer element assembly includes two similar piezoelectric plates,said plates each having two plane parallel surfaces, said piezoelectricplates being polarized so that opposite polarities appear at oppositesurfaces, electrodes on each of said plane parallel surfaces, rigidbonding means between one of said electroded surfaces of said firstplate and one of said electroded surfaces of said second plate, andseparate vibratile diaphragm means rigidly connected to each of theother electroded surfaces of said rigidly bonded plates.

10. The invention of claim 9 further characterized in that said twopiezoelectric plates are mechanically bonded together with electrodes ofopposite polarity facing each other and still further characterized inthat one electrical connection is made to said common facing electrodesand a second electrical connection is made in common to said oppositefacing electrodes.

11. The invention is claim 9 further characterized in that said twopiezoelectric plates are mechanically bonded together with theelectrodes of the same polarity on each plate facing each other andelectrically connected one to the other, and separate electricalconductor means connected to each of the other electrode surfaces oneach of said plates.

t l i

1. A pressure gradient electroacoustic transducer comprising atransducer element assembly having a pair of parallel surfaces, separatevibratile diaphragm means coupled to each of said parallel surfaces,means responsive to an application of substantially equal alternatingmechanical forces upon said diaphragms and saiD parallel surfaces forcausing said transducer element to generate two separate andsubstantially equal alternating voltages, and means for connecting saidtransducer elements for applying said separate and substantially equalalternating voltages in subtractive relationship one to the other,whereby an application of substantially equal mechanical forces uponsaid diaphragms produces a zero output.
 1. A pressure gradientelectroacoustic transducer comprising a transducer element assemblyhaving a pair of parallel surfaces, separate vibratile diaphragm meanscoupled to each of said parallel surfaces, means responsive to anapplication of substantially equal alternating mechanical forces uponsaid diaphragms and saiD parallel surfaces for causing said transducerelement to generate two separate and substantially equal alternatingvoltages, and means for connecting said transducer elements for applyingsaid separate and substantially equal alternating voltages insubtractive relationship one to the other, whereby an application ofsubstantially equal mechanical forces upon said diaphragms produces azero output.
 2. The invention of claim 1 wherein said electroacoustictransducer includes a tubular housing with an opening at each end ofsaid tubular housing, said transducer element assembly being positionedcentrally within said tubular housing, and said separate vibratilediaphragm means being attached to seal the open ends of said tubularhousing.
 3. An electroacoustic transducer comprising a tubular housingwith an opening at each end of said tubular housing, a transducerelement assembly having a pair of parallel surfaces, said transducerelement assembly being positioned centrally within said tubular housing,separate vibratile diaphragm means coupled to each of said parallelsurfaces, said separate vibratile diaphragm means being attached to sealthe open ends of said tubular housing, said transducer element assemblyincludes a tubular piezoelectric element having electrode surfaces onboth the inside and outside peripheral walls of said tubularpiezoelectric element, means responsive to an application ofsubstantially equal alternating mechanical forces upon said diaphragmsand said parallel surfaces for causing said transducer element togenerate two separate and substantially equal alternating voltages,means for applying said separate and substantially equal alternatingvoltages in subtractive relationship one to the other, whereby anapplication of substantially equal mechanical forces upon saiddiaphragms produces a zero output.
 4. The invention of claim 3 whereinat least one of said electrode surfaces on one of said peripheral wallsof said tubular element is divided into two substantially equal andelectrically separated areas.
 5. The invention of claim 4 furthercharacterized in that said voltage applying means is arranged to applythe voltages generated in each of said electrically separated areas in asubtractive relationship of one with respect to the other.
 6. Theinvention of claim 3 further characterized in that the electrode surfaceon one peripheral wall of said tubular piezoelectric element is dividedinto two electrically separated areas of substantially equal size andstill further characterized in that the electrode surface on theopposite peripheral wall is divided into four separated areas ofsubstantially equal size.
 7. The invention of claim 6 furthercharacterized in that the wall of said tubular piezoelectric element ispolarized in the same direction throughout the entire length of thetube.
 8. The invention of claim 7 further characterized in that fourseparate voltages are generated between the two separated electrodes onone peripheral wall and the four electrodes on the opposite peripheralwall of the piezoelectric tube, and means for combining the fourvoltages so that each is subtractive one from the other.
 9. Theinvention of claim 1 further characterized in that said transducerelement assembly includes two similar piezoelectric plates, said plateseach having two plane parallel surfaces, said piezoelectric plates beingpolarized so that opposite polarities appear at opposite surfaces,electrodes on each of said plane parallel surfaces, rigid bonding meansbetween one of said electroded surfaces of said first plate and one ofsaid electroded surfaces of said second plate, and separate vibratilediaphragm means rigidly connected to each of the other electrodedsurfaces of said rigidly bonded plates.
 10. The invention of claim 9further characterized in that said two piezoelectric plates aremechanically bonded together with electrodes of opposite polarity facingeach other and still further characterized in that one electricalconnection is made to said common facing electrodes and a secondelectrical connection is made in common to said opposite facingelectrodes.